= Two physical polarizations of a photon
{title2=$N_{\rm phys}=2$}
Two <first-class constraints> in Maxwell theory remove four phase-space variables from the eight variables of a four-component potential and its momenta. The remaining four phase-space variables form two <canonical pairs>. In <radiation gauge> these are transverse polarization modes; circular combinations have <helicity> $+1$ and $-1$. Auxiliary covariant-gauge components do not add physical <photon> states.
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